Gasoline, diesel and kerosene are the most commonly used liquid transportation fuels. It is known that aviation fuel is a specialized type of petroleum-based fuel used to power aircraft. It is generally of a higher quality than fuels used in less critical applications such as heating or road transport, and often contains additives to reduce the risk of icing or explosion due to high temperatures, amongst other properties. Most aviation fuels available for aircraft are kinds of petroleum spirit used in engines with spark plugs i.e. piston engines and Wankel rotaries or fuel for jet turbine engines which is also used in diesel aircraft engines.
Aviation fuels consist of blends of over a thousand chemicals, primarily hydrocarbons (paraffins, olefins, naphthenes, and aromatics) as well as additives such as antioxidants and metal deactivators, and impurities. Principal components include n-octane and isooctane. Like other fuels, blends of Aviation fuel used in piston engined aircraft are often described by their octane rating. The net energy content for present day aviation fuels depends on their composition. Some typical values are: Avgas, 43.7 MJ/kg or 31.0 MJ/L; Wide-cut jet fuel, 43.5 MJ/kg or 33.2 MJ/L, and Kerosene type jet fuel, 43.8 MJ/kg or 35.1 MJ/L.
Outside former communist areas, there are currently two main grades of turbine fuel in use in civil commercial aviation: Jet A-1 and Jet A, both are kerosene type fuels. There is another grade of jet fuel, Jet B which is a wide cut kerosene (a blend of gasoline and kerosene) but it is rarely used except in very cold climates. Jet A-1 is a kerosene grade of fuel suitable for most turbine engined aircraft. It is produced to a stringent internationally agreed standard, has a flash point above 38° C. (100° F.) and a freeze point maximum of −47° C. Jet A is a similar kerosene type of fuel as Jet A-1 but with a higher freeze point maximum (−40° C.). Jet B is a distillate covering the naphtha and kerosene fractions. It can be used as an alternative to Jet A-1 where its better cold weather performance is important.
JP-4 is the military equivalent of Jet B with the addition of corrosion inhibitor and anti-icing additives. JP-5 is a high flash point kerosene. Finally, JP-8 is the military equivalent of Jet A-1 with the addition of corrosion inhibitor and anti-icing additives.
As mentioned, aviation fuel typically contains additives added to the fuel in very small quantities, usually measurable only in parts per million, to provide special or improved qualities. The quantity to be added and approval for its use in various grades of fuel is strictly controlled by the appropriate specifications. A few additives in common use are as follows:    1. Anti-knock additives reduce the tendency of gasoline to detonate. Tetra-ethyl lead (TEL) is the only approved anti-knock additive for aviation use and has been used in motor and aviation gasolines since the early 1930s.    2. Anti-oxidants prevent the formation of gum deposits on fuel system components caused by oxidation of the fuel in storage and also inhibit the formation of peroxide compounds in certain jet fuels.    3. Static dissipater additives reduce the hazardous effects of static electricity generated by movement of fuel through modern high flow-rate fuel transfer systems. Static dissipater additives do not reduce the need for ‘bonding’ to ensure electrical continuity between metal components (e.g. aircraft and fuelling equipment) nor do they influence hazards from lightning strikes.    4. Corrosion inhibitors protect ferrous metals in fuel handling systems, such as pipelines and fuel storage tanks, from corrosion. Some corrosion inhibitors also improve the lubricating properties (lubricity) of certain jet fuels.    5. Fuel System Icing Inhibitors (Anti-icing additives) reduce the freezing point of water precipitated from jet fuels due to cooling at high altitudes and prevent the formation of ice crystals which restrict the flow of fuel to the engine. This type of additive does not affect the freezing point of the fuel itself. Anti-icing additives can also provide some protection against microbiological growth in jet fuel.    6. Metal de-activators suppress the catalytic effect which some metals, particularly copper, have on fuel oxidation.    7. Biocide additives are sometimes used to combat microbiological growths in jet fuel, often by direct addition to aircraft tanks; as indicated above some anti-icing additives appear to possess biocidal properties.    8. Thermal Stability Improver additives are sometimes used in military JP-8 fuel, to produce a grade referred to as JP-8+100, to inhibit deposit formation in the high temperature areas of the aircraft fuel system.
As mentioned above, alcohol, alcohol mixtures and other alternative fuels may be used experimentally but are not generally available.
Although alternative aviation fuels are not general available, there are nonetheless many patents and scientific articles on (aviation) fuels containing for instance furan derivatives such as furfural, furfuryl alcohol, methylfuran, and dimethylfuran. The most important examples are summarized below.
Already in U.S. Pat. No. 2,599,338, fuel compositions are described containing tetrahydro furfuryl alcohol. The presence of this component reduced the loss in speed due to icing both in motor fuel and in an aviation fuel.
Recently, in WO2008013922, fully renewable engine fuels were described that are said to be derived completely from biomass sources. The fully renewable engine fuel is comprised of one or more low carbon number esters derived from ethanol, one or more pentosan-derivable furans, one or more aromatic hydrocarbons, one or more C4-C10 straight chain alkanes derivable from polysaccharides, and one or more bio-oils. In addition, the fuel may contain triethanolamine. Such a lower octane renewable fuel may be utilized, for example, in automobile fuel, 100 LL aviation fuel applications, and turbine engine applications. These ethanol-based, fully renewable fuels may be formulated to have a wide range of octane values and energy, and may effectively be used to replace 100LL aviation fuel (known as “aviation gasoline” or “AvGas”), as well as high octane, rocket, diesel, and turbine engine fuels.
As discussed in this reference, the inherent energy contained within gasoline is directly related to mileage, not to octane number. Automobile gasoline has no energy specification, hence no mileage specification. In contrast, aviation fuels, a common example being 100 LL (100 octane low lead), do have an energy content specification. This translates to aircraft range and to specific fuel consumption. Aircraft cannot compromise range. For this reason, energy content is equally important as MON values. It should be realized, however, that the aforementioned ethanol-based fuels are produced through fermentation of food crops and such and therefore compete with the production of food products. The generation of biofuel from food crops is widely discussed, given the pressure it will put on land, prices of food and the hunger problem.
Furan derivatives have indeed been used for various purposes in fuel compositions. In e.g., US20060180786A tetrahydrofurfuryl acetate and other derivatives have been described as pour point depressant for jet fuel/diesel fuel compositions. More specifically, this reference provides a method for lowering the pour point of biodiesel, said method comprising preventing or reducing crystallization of fatty acids, esters or alkaline salts of fatty acids in the biodiesel product by adding to said biodiesel an effective crystallization reducing or preventing amount of a composition, said composition comprising a.o. tetrahydrofurfuryl acetate and tetrahydrofurfuryl tetrahydrofuroate. Indeed, as antifreeze component, furfuryl alcohol has already been described in U.S. Pat. No. 2,229,215.
Furan derivatives have been suggested also for use in hypergolic propellants. A hypergolic propellant is either of the two rocket propellants used in a hypergolic rocket engine, which spontaneously ignite when they come into contact. The two propellants are usually termed the “fuel” and the “oxidizer”. Although hypergolic propellants tend to be difficult to handle, a hypergolic engine is easy to control and very reliable. In common usage, the terms “hypergol” or “hypergolic propellant” are often used to mean the most common such propellant combination, hydrazine plus nitrogen tetroxide, or their relatives. Examples of furan derivatives in hypergolic propellants may be found in U.S. Pat. No. 4,316,359, U.S. Pat. No. 2,994,191, U.S. Pat. No. 2,993,334, and U.S. Pat. No. 2,874,535.
U.S. Pat. No. 4,339,245 describes a gasoline composition containing at least one antiknock compound selected from the group consisting of furfuryl acetate, ethyl furfurylacrylate, methyl furoate, and ethyl furoate. This case is not specific to aviation fuels. Similar cases on antiknock components, cetane improvers, or component to improve the octane rating may be found in U.S. Pat. No. 3,359,087, U.S. Pat. No. 3,021,204, EP1321504, WO2005087901, U.S. Pat. No. 5,925,152, U.S. Pat. No. 5,697,987 (which describes 2-methyltetrahydrofuran as an anti-knock component for spark ignition motor fuel composition at levels between 15-55%), U.S. Pat. No. 4,522,630, U.S. Pat. No. 2,321,311 (which discloses a motor fuel composition comprising a mixture of gasoline hydrocarbons adapted as a base fuel for spark-ignition engines improved in antiknock value by an addition of a substantial amount at least 1% by volume of a heterocyclic compound containing a furane nucleus and selected from the group consisting of furane, alkyl furanes, furfuryl alcohols, furfuryl amines, and the saturated derivatives thereof, said heterocyclic compound boiling in the range of said base fuel), US2003110684 and many other references.
From US2003154649 a method is known for reducing the vapor pressure of a C3 to C12 hydrocarbon-based motor fuel composition for a conventional spark ignition internal combustion engine comprising combining: (a) a hydrocarbon component comprising C3 to C12 hydrocarbon fractions; (b) an ethanol component comprising fuel grade ethanol, said ethanol component comprising 0.1% to 20% of the composition by volume; (c) an oxygen-containing heterocyclic compound having 5 to 8 carbon atoms selected from the group consisting of tetrahydrofurfuryl alcohol, tetrahydrofurfuryl acetate, dimethyltetrahydrofuran, tetramethyltetrahydrofuran.
US2002053161 discloses the use of oxygenated compounds derived from tetrahydrofurfuryl as additives or formulation bases of gas-oils and leading to a significant lowering of particle emissions. The oxygenated compound has the formula THF—C—X—R in which —X— is chosen from —O— and —O—(CH2-O)n (n=1-20) and R is chosen from alkyl groups containing from 1 to 30 carbon atoms; and groups containing a tetrahydrofurfuryl unit (isopropyl, isobutyl, tertbutyl and tertamyl groups).
Unfortunately, such furane derivatives are relatively expensive to make, they are relative unstable and/or their physical/chemical properties (energy density, melting point, boiling points flash points viscosity) are suboptimal.
The prior art comprises additional references on the use of certain furfuryl derivatives as yarn conditioners (U.S. Pat. No. 2,196,747 and U.S. Pat. No. 2,153,135), as component of a heat pump (GB1539287) and as topic for study in various reactions (W. R. Kirner: “Alpha-Tetrahydrofurfuryl Chloride and Alpha-Tetrahydrofurfuryl Ethers”, JACS, vol. 52, 1930, pages 3251-3256; Cope et al: “Aminoalcohols Containing the 8-Oxa-3-azabicyclo[3.2.1.]octane Ring System and Their Benzoates” JACS, vol. 77, 1955, pages 393-396; E. Taskinen: “Relative Thermodynamic Stabilities of 2-(Methoxymethylene)tetrahydrofuran and 5-Methoxymethyl-2,3-Dihydrofuran” J. of Physical Organic Chemistry, vol. 8, 1995, pages 1-4; Schiavo et al: “Catalytic hydrogenation of 5-(hydroxymethyl)furfural in aqueous medium”, Bulletin de la Soc. Chimique de France, 1991, pages 704-711). As of the use of these furfuryl derivatives as components in fuels and particular in aviation fuels nothing is mentioned in this art.
Likewise, the substances 2,5-bis-methoxymethyltetrahydrofuran (Beilstein registry number 106201) and 2-hydroxymethyl-5-methoxymethyl-tetrahydrofuran (Beilstein registry number 105046) are known, but made by complicated processes without disclosing their use as fuel or fuel component (cf. XP002498014 and XP002498015).
Strongly increasing fossil fuel prices, security of supply issues and growing concerns about greenhouse gas emissions have intensified the search for and use of Bioenergy applications (solar, wind, biomass, geothermal etc). An important area is the transportation sector but unfortunately on the short term only limited renewable energy solutions are available.
From an environmental perspective hydrogen would be a very attractive fuel. Unfortunately, hydrogen is a very inconvenient energy storage medium. Thus, the density of hydrogen at room temperature and pressure is 0.00009 kg/m3. The density at 700 bar amounts to 57.5 kg/m3, with an energy density of Qv=120 MJ/kg. The density of gasoline is 740 kg/m3, Qv=44 MJ/kg. Although per kg, hydrogen has a 120/44=2.7× higher energy density than gasoline, because of the low mass per volume, even at 700 bars hydrogen has only (2.7*(57.5/740)) 21% of the energy content of gasoline. In addition, compression to 700 bar requires 20% of fuel energy. Hydrogen can also be stored as a liquid in a cryogenic tank. However, when stored at −253° C. the density is still only 70 kg/m3. This liquefaction requires 40% of fuel energy. The required fuel tanks (for both pressurization and liquefaction) are heavy and expensive. A cryogenic tank to store 8 kg fuel weighs 120 kg—high pressure tanks are significantly heavier! A major disadvantage of liquefaction is that half the fuel boils off in 9 days!
A further solution has been to store hydrogen in mineral or organic materials. Metallocarboranes have been able to reversibly store 50 kg/m3 hydrogen (NREL annual hydrogen report 2007). These materials are however prohibitively expensive. So far the strategies for hydrogen storage via reaction or complexation with “host” materials required release of hydrogen upon use.
Another fuel considered attractive from an environmental perspective is biofuel (i.e., fuels of bioorganic origin as opposed to fuels of petroleum origin).
Biofuel can be broadly defined as solid, liquid, or gas fuel consisting of, or derived from recently dead biological material, most commonly plants. This distinguishes it from fossil fuel, which is derived from long dead biological material. Biofuels are used globally. The most common use for biofuels is automotive transport. The use of renewable biofuels in lieu of fossil fuels is said to reduce greenhouse gas emissions and increase energy security.
One of the greatest technical challenges is to develop ways to convert biomass energy specifically to liquid fuels for transportation. To achieve this, the two most common strategies are:    1. To grow sugar crops (sugar cane, and sugar beet), or starch (corn/maize), and then use yeast fermentation to produce ethanol (ethyl alcohol).    2. To grow plants that (naturally) produce oils, such as palm, rapeseed, soy, algae, or jatropha. When these oils are heated, their viscosity is reduced, and they can be burned directly in a diesel engine, or the oils can be chemically processed to produce fuels such as biodiesel.Increased demand for biofuels, particularly in America and Europe has led to deforestation and food shortages. This is increasingly making biofuels into a political issue throughout the world. Recently, inventors of Furanix Technologies B.V. have developed an alternative route to biofuels, based on 5-alkoxymethyl furfural derivatives that are prepared by reacting a fructose and/or glucose-containing starting material with an alcohol in the presence of a catalytic or sub-stoichiometric amount of heterogeneous acid catalyst. See WO2007104514. The catalysts may be employed in a continuous flow fixed bed or catalytic distillation reactor. The ethers can be applied as a fuel or fuel additive.
As indicated in this prior patent application, the conversion of one glucose molecule by fermentation into two molecules of ethanol and two molecules of CO2 has drawbacks especially in view of atom economy, the low energy density of ethanol (7.7 kWh/kg or 6.1 kWh/L), its relative low flash point (13 degrees Celsius), hygroscopic nature and corrosiveness.
For fuel and aviation fuel in particular, products of increased energy density, flash point, and melting point are required. Ideally, these products should be based on biomass or biomass products. On the other hand, the preparation of these (aviation) biofuels should be economically feasible and energetically acceptable. The products should be at least as good as methylfuran and dimethylfuran, that have been proposed as aviation fuel in the past, but which production-routes were non-economic to date.
Interestingly, the current inventors have found a new class of compounds that may be used as fuel and/or as fuel additive. These compounds may be produced by proven technologies, using starting materials based on biomass or biomass products.
In fact the current inventors have realised that unlike the products resulting from mineral oil, biomass derived furans are very high in unsaturation. This enables the use of these compounds as a cheap hydrogen storage medium, without the challenging hydrogen release requirement! Both the hydrogen as well as the furan “storage host” can be used as an economic fuel. Moreover, it has now been realized to prepare biomass derived fuel components that meet the requirements concerning flash point and freeze point maximum whilst providing a high energy density. In other words, the new fuel components are uniquely suitable for use as aviation fuel.